10 Application performance Interview Questions and Answers for site reliability engineers

flat art illustration of a site reliability engineer

1. What inspired you to pursue a career in site reliability engineering?

Throughout my career as a software engineer, I've always been interested in solving complex problems and ensuring that my applications run smoothly. However, it wasn't until about three years ago when I was working at a fast-growing startup that I truly discovered my passion for site reliability engineering.

As the company rapidly expanded, we encountered numerous challenges related to scaling and maintaining our infrastructure. I was tasked with helping to address these issues, and I quickly realized that I had a knack for it. I enjoyed the challenge of optimizing our systems and finding ways to prevent downtime.

Over time, I began to focus more and more on site reliability engineering, attending industry conferences and reading up on best practices. I also started implementing new technologies and processes to improve our systems, with measurable results. For example, I implemented automated monitoring tools, which reduced our average downtime by 50% in just six months.

  1. Reduced average downtime by 50% in six months
  2. Implemented automated monitoring tools to improve system reliability
  3. Attended industry conferences and read up on best practices on site reliability engineering

Overall, the challenges and successes I experienced while working on our infrastructure at the startup inspired me to pursue a career in site reliability engineering. I enjoy the combination of problem-solving, technical expertise, and strategic thinking required for the role, and I'm committed to continuing to learn and grow in this field.

2. What do you think are the biggest challenges involved in improving application performance?

Improving application performance is a complex process with numerous challenges. One of the biggest challenges is identifying the root cause of performance issues. Many factors can impact application performance, including network latency, database performance, and code inefficiencies. Without proper monitoring tools or sufficient logging, pinpointing the cause of these issues can be a time-consuming and difficult task.

Another significant challenge is optimizing performance across different platforms and devices. With the proliferation of mobile devices and web browsers, developers must ensure that their applications function seamlessly across all of these platforms. This requires extensive testing and optimization, as well as an intimate knowledge of the various technologies and frameworks being used.

Performance testing and monitoring are also critical components of improving application performance. Load testing can help identify bottlenecks in the system, while real-time monitoring can detect and alert developers to performance issues as they occur. With the massive amounts of data being generated by these tools, it can be challenging to identify meaningful insights that can drive performance improvements.

  1. Identifying the root cause of performance issues
  2. Optimizing performance across different platforms and devices
  3. Performance testing and monitoring

Overcoming these challenges requires a combination of technical expertise, a data-driven approach, and a commitment to ongoing testing and optimization. By leveraging the latest tools and techniques, developers can ensure that their applications deliver the best possible performance for users, leading to improved user satisfaction, engagement, and revenue.

3. Tell me about a time when you identified a bottleneck in a complex system and how you addressed it.

During my time at XYZ Corporation, I was tasked with optimizing the performance of a complex system that was experiencing slow response times. During my analysis, I noticed that a particular API was causing a significant bottleneck in the overall system.

  1. I reviewed the logs to identify the source of the problem and discovered that the API was being called excessively, causing an overload on the server.
  2. I contacted the team responsible for the API and worked with them to optimize the endpoint by implementing caching and reducing the number of unnecessary calls.
  3. Additionally, I worked with the developers to update the system's architecture by implementing Load Balancers and distributed caching systems to handle the increased traffic.
  4. Lastly, I set up proactive monitoring to alert the team in case of a similar issue occurring in the future.

As a result of my intervention, the API response time improved by 75% in the first quarter, reducing the overall request time of the system by 50%. The proactive monitoring also made the team aware of future issues, allowing them to take a more proactive approach to optimize the system.

4. How do you keep up to date with the latest technologies and tools related to application performance?

Keeping up to date with the latest technologies and tools related to application performance is crucial to my success as a developer. Here are the ways I do it:

  1. Newsletters: I subscribe to newsletters from industry leaders such as the Performance Calendar, Google Developers, and Web Performance Today. Keeping up-to-date with the latest news and trends in my inbox helps me stay informed without dedicating too much time.
  2. Blogs: I follow blogs of industry experts and thought leaders such as High Scalability and Perf Matters. They provide in-depth articles and case studies on the latest tools, technologies, and best practices in application performance.
  3. Conferences and Webinars: I regularly attend virtual conferences and webinars on topics such as JavaScript frameworks, cloud computing, and DevOps to stay updated on the latest trends in the industry. At a recent virtual conference, I attended a session on performance testing for microservices and was able to implement the learnings into my project, which improved the load time by 25%.
  4. Training and Certification: I stay up-to-date through online courses on platforms such as Udemy and Coursera, such as recently completing the Google Cloud Platform Fundamentals course that boosted the performance of a cloud-based application by 20%. Additionally, I have recently taken the AWS Certified Developer - Associate exam and received a score of 92%.

By using these approaches, I have been able to improve the application performance of multiple projects for clients. For instance, by staying up to date with CloudFlare and implementing browser caching of assets and optimizing images, I was able to improve the site’s page load speed by 50%.

5. What is your approach to monitoring and analyzing system performance?

My approach to monitoring and analyzing system performance starts with setting up proper monitoring tools and alerts. I use a combination of CloudWatch and ELK Stack to monitor server logs, application logs, and system metrics. The tools are configured to notify me and my team whenever the system metrics exceed certain thresholds or whenever there are any error logs.

I also prioritize analyzing performance bottlenecks identified by our monitoring tools. A few months ago, I noticed an increase in response time of an application during peak hours. I analyzed our database queries and realized that some of the queries were taking longer to execute than expected. I decided to optimize the queries, and the response time improved drastically, reducing the average response time from 1.5 seconds to 0.5 seconds.

In addition, I keep track of user experience metrics to ensure that the system is performing optimally. Recently, I implemented Apdex score in our system, and after a few weeks of tracking, we identified some areas where the user experience was suboptimal. We addressed these areas, resulting in a 20% increase in the Apdex score.

Overall, my approach to monitoring and analyzing system performance is to set up the right monitoring tools, analyze any bottlenecks identified, and keep track of user experience metrics to ensure that the system performs optimally.

6. Tell me about a project where you had to troubleshoot a production issue that affected performance.

During my time working as a software engineer at XYZ Company, I was tasked with troubleshooting a production issue that was affecting performance. Our application was experiencing slow response times which was causing frustration to our end-users. My team and I immediately took action to identify the root cause of the issue and implemented a solution to resolve it.

  1. We analyzed server logs and found that a particular query was taking a long time to execute. We also noticed that the query was being executed multiple times due to a coding error.

  2. To address this issue, we decided to restructure the database tables to improve performance. We also looked for any ways we could optimize the query. After a thorough analysis, we were able to identify a few areas where we could apply indexing and caching to speed up the query.

  3. As a result of these changes, the query execution time was reduced by 70%! This resulted in a significant improvement in application performance with response times returning to acceptable levels.

  4. We also implemented a monitoring system to ensure that similar errors could be caught before they caused performance problems in the future. We used a combination of tools that would notify us of any anomalies as well as record information about performance metrics over time.

The results were clear - our end-users reported a noticeable improvement in application performance, with response times returning to acceptable levels. Our monitoring system also allowed us to proactively identify and rectify any issues before they could escalate into performance problems.

7. What steps do you take to ensure system reliability and availability during peak traffic?

During peak traffic, ensuring system reliability and availability is crucial to prevent any potential downtime or crashes. To accomplish this, I follow these steps:

  1. Perform load testing prior to peak traffic periods to identify any bottlenecks or weak points in the system.
  2. Regularly monitor performance metrics such as CPU utilization, disk I/O, and network usage to detect any anomalies and proactively take action.
  3. Implement horizontal scaling, where resources are added dynamically to handle increased traffic, and vertical scaling, where resources are upgraded to handle larger workloads.
  4. Utilize caching mechanisms such as CDN, memcached or Redis to reduce database load and improve response time.
  5. Use database optimization techniques such as indexing, query optimization, and data partitioning to improve database performance.
  6. Implement failover mechanisms and redundancy in multiple availability zones to ensure the system can handle unexpected failures and maintain high availability.
  7. Perform regular backups of critical data, and test the backup restoration process to ensure data availability in case of failures or data loss.
  8. Implement cloud monitoring services to receive alerts and take action in real-time, whenever any issues arise.
  9. Use a Content Delivery Network (CDN) to distribute content and improve response times.
  10. Conduct regular security checks and update security measures to prevent security breaches.

By following these steps, I was able to ensure the reliable and smooth functioning of the system, even during peak traffic periods. For example, during a peak sales event, the system was able to handle over 10 times the normal traffic with response times under 600 ms, without any downtime or crashes.

8. How do you prioritize and manage your workload when handling multiple performance-related issues?

When handling multiple performance-related issues, I prioritize my workload based on the impact or severity of the issue. Typically, I focus on resolving the high-severity issues first, as they have the most significant impact on the overall system or application performance.

  1. First, I identify the priority of each performance-related issue based on the criticality of the system affected.
  2. Then, I categorize the issues into three categories: high, medium, and low based on their impact.
  3. Next, I work on the high-priority issues first, and conduct root cause analysis to understand the reason for their occurrence to prevent similar performance issues from occurring in the future.
  4. After the high-priority issues have been resolved, I move on to the medium-priority issues, and then the low-priority ones.

To manage my workload efficiently, I also use task management tools like Jira or Trello to track my progress and communicate with other team members. I ensure that I provide regular updates to my team and stakeholders on the status of each issue, including any potential roadblocks or delays.

During my time as a performance engineer at XYZ Corp, I employed this approach to handle multiple performance issues affecting some critical application components. I was able to reduce the number of production issues by 30% within the first two months by prioritizing and managing my workload effectively.

9. Walk me through your methodologies for identifying and mitigating security threats related to application performance.

When it comes to identifying and mitigating security threats related to application performance, my approach involves several key steps:

  1. Conducting a thorough risk assessment: Before I can begin to address potential security threats, I first need to understand what those threats might be. To do this, I conduct a comprehensive risk assessment that includes looking at factors such as the application's architecture, code quality, and third-party integrations.

  2. Implementing proactive monitoring systems: Once I have a better understanding of the potential security risks, I set up monitoring systems to track key performance indicators (KPIs) that can help me identify any unusual activity. For example, I might set up alerts to notify me if there is a sudden spike in error rates or if response times exceed a certain threshold.

  3. Using AI to detect anomalies: In addition to setting up monitoring systems, I also leverage AI and machine learning technologies to identify potential security threats. These systems can analyze large amounts of data to detect patterns and anomalies that might be missed by human analysts.

  4. Implementing security protocols and best practices: To mitigate any potential security threats, I implement robust security protocols and best practices that are customized to the specific requirements of the application. This might include things like encryption, firewalls, and access controls.

  5. Regularly testing and updating security systems: Finally, I regularly test and update the security systems in place to ensure that they remain effective against new and emerging threats. This might involve running penetration tests, upgrading software packages, or implementing new authentication methods.

By following this methodology, I am able to effectively identify and mitigate security threats related to application performance. For example, in my previous role as a lead developer for a fintech company, I was responsible for overseeing the development of an online payments platform. By implementing the steps outlined above, we were able to prevent all major security incidents and had a 99.99% uptime rate over the course of a year.

10. What metrics do you track to measure system performance and how do you use that data to improve performance?

As part of monitoring and improving system performance, I track multiple metrics, including:

  1. Response Time: I measure the time it takes for the system to respond to a user request. Through a benchmarking tool, I can set a response time threshold, and if it goes over, I'll look for ways to optimize the system.
  2. Error Rates: I keep an eye on the number of errors or failures the system encounters. Through alerts, I can quickly pinpoint errors and work on resolving them as soon as possible.
  3. CPU Usage: I monitor the system CPU usage to identify any spikes or bottlenecks that could result in poor system performance. If CPU usage spikes, I'll investigate and try to resolve the issues, whether it be inefficient code or a need for additional hardware resources.
  4. Memory Usage: Another crucial metric is memory usage, which can impact system performance. I monitor how much memory the system uses at any given time and optimize resources accordingly to keep performance high.

Using data from these metrics, I can see which areas of the system need improvement and work with the development team to optimize code, upgrade hardware, or take other action to ensure that users can smoothly interact with the application. For example, I once noticed a spike in response time, which led to more user frustration and complaints. With further analysis, I discovered that the cause was an inefficient database query. I worked with the team to rewrite the query, and as a result, the response time improved by 40%, and user satisfaction ratings increased.


Congratulations on getting familiar with 10 top-notch application performance interview questions and answers for 2023! Now, the next steps for you should be to write a cover letter and prepare a remarkable CV to impress potential employers. Don't forget to use our comprehensive guides on how to write a cover letter and resume for site reliability engineers. Our cover letter guide is available here:

Ace Your Site Reliability Engineer Cover Letter

and the guide on how to write a killer resume is available here:

Create An Impressive Site Reliability Engineer Resume

. Finally, if you are on the lookout for a remote job, then our website's job board is for you. Don't miss out on the chance to explore our remote site reliability engineer jobs available here:

Browse for Remote Site Reliability Engineer Jobs

. Good luck with your job search and don't forget to let us know when you land your dream job!
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